1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for utilizing a switching power converter and determining an approximate zero crossing of an input voltage to the switching power converter.
2. Description of the Related Art
Many electronic systems include circuits, such as switching power converters that interface with a dimmer. The interfacing circuits deliver power to a load in accordance with the dimming level set by the dimmer. For example, in a lighting system, dimmers provide an input signal to a lighting system. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers generate a digital or analog coded dimming signal that indicates a desired dimming level. For example, some analog based dimmers utilize a triode for alternating current (“triac”) device to modulate a phase angle of each cycle of an alternating current (“AC”) supply voltage. “Modulating the phase angle” of the supply voltage is also commonly referred to as “phase cutting” the supply voltage. Phase cutting the supply voltage reduces the average power supplied to a load, such as a lighting system, and thereby controls the energy provided to the load.
Once a triac-based dimmer begins conducting during a cycle of an alternating current (“AC”) supply voltage, to prevent the triac from disadvantageously, prematurely disconnecting during mid-cycle of the supply voltage, the switching power converter draws a minimum current referred to as a “hold current”. As long as an input current to the switching power converter is greater than or equal to the hold current, the triac-based dimmer should not prematurely disconnect. For a leading edge dimmer, a premature disconnect occurs when the dimmer begins conducting and stops conducting prior to reaching a zero crossing of the supply voltage. Premature disconnects can cause problems with the lighting system, such as flicker and instability.
FIG. 1 depicts a lighting system 100 that includes a leading edge, phase-cut dimmer 102. FIG. 2 depicts ideal, exemplary voltage graphs 200 associated with the lighting system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VIN from voltage supply 104. The supply voltage VIN, indicated by voltage waveform 202, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. A leading edge dimmer 102 phase cuts leading edges, such as leading edges 204 and 206, of each half cycle of supply voltage VIN. Since each half cycle of supply voltage VIN is 180 degrees of the supply voltage VIN, the leading edge dimmer 102 phase cuts the supply voltage VIN at an angle greater than 0 degrees and less than 180 degrees. Generally, the voltage phase cutting range of a leading edge dimmer 102 is 10 degrees to 170 degrees.
The input signal voltage VΦ—IN to the lighting system 100 represents a dimming level that causes the lighting system 100 to adjust power delivered to a lamp 122, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp 122. The leading edge dimmer 102 can be any type of leading edge dimmer, such as a triac-based leading edge dimmer available from Lutron Electronics, Inc. of Coopersberg, Pa. (“Lutron”). A triac-based leading edge dimmer is described in the Background section of U.S. patent application Ser. No. 12/858,164, entitled Dimmer Output Emulation, filed on Aug. 17, 2010, and inventor John L. Melanson.
The phase cut dimmer 102 supplies the input voltage VΦ—IN as modified by the phase cut dimmer 102 to a full bridge diode rectifier 106. The full bridge rectifier 106 supplies an AC rectified voltage VΦR—IN to the switching power converter 108. Thus, since the input voltage VΦ—IN is derived from the input supply voltage VIN, the rectified input voltage VΦR—IN is also derived from the input supply voltage VIN. Capacitor 110 filters high frequency components from rectified voltage VΦR—IN. To control the operation of switching power converter 108, controller 111 generates a control signal CS0 to control conductivity of field effect transistor (FET) switch 112. The control signal CS0 is a pulse width modulated signal. Control signal CS0 waveform 114 represents an exemplary control signal CS0. Each pulse of control signal CS0 turns switch 112 ON (i.e. conducts), and the inductor current iL increases, as shown in the exemplary inductor current waveform 115, to charge inductor 116 during a charging phase TC. Diode 118 prevents current flow from link capacitor 120 into switch 112. When the pulse ends, the inductor 116 reverses voltage polarity (commonly referred to as “flyback”), and the inductor current iL decreases during the flyback phase TFB, as shown in inductor current waveform 115. The inductor current iL boosts the link voltage across the link capacitor 120 through diode 118.
The switching power converter 108 is a boost-type converter, and, thus, the link voltage VLINK is greater than the rectified input voltage VΦR—IN. Controller 111 senses the rectified input voltage VΦR—IN at node 124 and senses the link voltage VLINK at node 126. Controller 111 operates the switching power converter 108 to maintain an approximately constant link voltage VLINK for lamp 122, provide power factor correction, and correlate the output current iOUT with the phase cut angle of the rectified input voltage VΦR—IN. Lamp 132 includes one or more light emitting diodes.
It is desirable to improve interfacing with triac-based dimmers.